The use of resonant piezoelectric crystals as monolithic filters is well known, having been described in various publications including a text book entitled, Modern Crystal And Mechanical Filters, D. F. Sheahan and R. A. Johnson, EDS, published by the IEEE Press, 1977. Section II-A at pages 192-193 provides an historical overview of such devices and describes the effect of energy trapping, i.e., the confinement of the mechanical vibrating energy of a resonator to the area immediately beneath and between the electrodes.
Furthermore, various cuts of quartz used as monolithic crystal filters are also known. The AT cut of quartz has been extensively used although any singly rotated cut of quartz or other material such as berlinite (AlPO.sub.4) can be used. Recently, however, the use of doubly rotated cuts has been investigated and reported, for example, by the present inventor in Volume 13, Chapter 5, "Doubly Rotated Thickness Mode Plate Vibrators", of a text book entitled, Physical Acoustics, (W. B. Mason and R. N. Thurston, EDS), Academic Press, New York, 1977, pp. 115-181. An inherent problem exists with doubly rotated monolithic crystal filters using SC cuts of quartz in that the direction of the projection, in the plane of the plate, of the particle motion (normal coordinate) also referred to as the eigenvector projection, of the desired mode producing the filter response is at an angle relative to the doubly rotated axes in the plane of the plate. This has the effect of producing undesired mode couplings, improper phase and insertion loss responses, as well as providing less latitude of the design with respect to energy trapping in filters.
Accordingly, it is an object of the present invention to provide an improvement in piezoelectric crystal filters.
It is a further object of the invention to provide a piezoelectric monolithic crystal filter having improved frequency selectivity.
It is another object of the present invention to provide a monolithic crystal filter which is compensated for frequency shifts over a prolonged length of time resulting from the effects of electrode stress.
It is yet another object of the invention to provide a monolithic crystal filter which is compensated for bandwidth and insertion loss changes resulting from temperature transients over relatively short time periods.
It is yet a further object of the invention to provide for an improvement in the operational characteristics of monolithic crystal filters using doubly rotated plates of piezoelectric material.
And still a further object of the invention is to provide for improved monolithic crystal filters using an SC cut of quartz.